Antennas may be used for a variety of purposes, such as for wireless communications devices including broadcast receivers, pagers, two-way radios, or radio location devices (“ID tags”). The cellular telephone is an example of a wireless communications device, which is nearly ubiquitous. A relatively small size, increased efficiency, and a relatively broad radiation pattern are generally desired characteristics of an antenna for a portable radio or wireless device. It may be desirable for an antenna to communicate at a given frequency with desired characteristics, such as bandwidth, polarization, or gain pattern. Omnidirectional antennas, radiating a circular shaped pattern about the horizon may be preferred as they reduce the need for antenna orientation or aiming.
One particularly popular type of antenna is a dipole antenna. A dipole antenna is a radio antenna that includes a center-fed driven element. Two conductors (e.g. rods or wire) are oriented collinear with each other (in line with each other), with a small space between them. Radio frequency (RF) voltage is applied to the antenna at the center, between the two conductors. The half power or 3 dB gain bandwidth of a thin wire (diameter <λ/50) half wave dipole antenna may be about 13.5 percent, the 2 to 1 voltage standing wave ratio (VSWR) bandwidth about 4.5 percent, and the loaded circuit Q about 14.8, which may not be adequate. Realized gain versus frequency plots are typically quadratic in shape near the half wave dipole resonance. VSWR response is also quadratic.
To achieve desired antenna characteristics, the size and shape of an antenna, for example, of a dipole antenna may be adjusted. U.S. Pat. No. 4,352,109 to Reynolds et al. discloses an inner end-supportable, center driven dipole antenna having a two part outer or upper element or rod. The antenna is supported at its lower or inner end to be vertical or at an angle thereto. The rod is fixed by a clamp so that the upper element has the length of ⅝ wavelength. The upper element is connected to an inner or lower element portion of a conductive support mast. The lower element or pole also has a length of ⅝ wavelength so that the total length of the dipole is 1¼ wavelengths. The dipole connecting arrangement includes an impedance matching network. The rods are conductors. The lower end of one rod is flared to fit within a conical portion of a dielectric support and extends upwardly through a cylindrical opening in which it is snugly fitted so as to be watertight.
U.S. Pat. No. 6,483,471 to Petros discloses a sleeve dipole. A tubular dipole includes a coaxial cable, an inner conductor, and a balun with the quarter-wave metal sleeve. The inner conductor extends from the top of the balun and is coupled to a quarter-wave hollow metal tube. In one embodiment, a multiple tubular dipole antenna includes comprises a coaxial cable having an inner conductor and an outer conductor both running vertically and concentrically through a quarter-wave metal sleeve. The antenna further includes a shorted end formed from the connection of the outer conductor of the coaxial cable to an end of the quarter-wave metal sleeve. Additionally, a quarter-wave hollow metal tube is connected to the inner conductor of the coaxial cable extending from the end of the quarter-wave metal sleeve. An additional dipole antenna is configured substantially concentrically above the quarter-wave hollow metal tube using another quarter-wave metal sleeve and a hollow metal tube. The antenna also includes a shorted end formed from the connection of the outer conductor of the coaxial cable to an end of the quarter-wave metal sleeve. The hollow metal tube is connected to the inner conductor of the coaxial cable extending from the end of the quarter-wave metal sleeve.
U.S. Pat. No. 8,081,130 to Apostolos et al. is directed to a broadband whip antenna. More particularly, Apostolos et al. discloses a shortened multi-band antenna that includes in-line dipoles, selected elements of which having shielded meanderline chokes to be able to switch from an extended dipole at the lower VHF frequencies to a shortened dipole for the UHF band.
Referring to FIG. 1, a prior art dipole antenna 120 is now described. The antenna 120 may be configured to achieve a bandwidth gain of four. A coaxial antenna feed 140 extends through a second tubular dipole element 150 having a proximal end 151 and an opposing distal end 152. The second tubular dipole element 150 includes first and second spaced apart tubular segments 153, 154 aligned in spaced apart relation. The coaxial antenna feed 140 includes an inner conductor 141, an outer conductor 142, and a dielectric 143 therebetween. The outer conductor 142 of the coaxial antenna feed 140 is coupled to the proximal and distal ends 151, 152 of the second dipole element 150. The inner conductor 141 extends outwardly from the distal end 152 of the second tubular dipole element 154 and couples to a first tubular dipole element 130 at a proximal end 131.
Further improvements to dipole antennas may be desired. For example, it may be particularly desirable to increase ease of assembly while maintaining an increased frequency response.